Basically, it is a crap shoot, but there are ways where loaded dice can be detected. After all, it’s more likely a 7 will come up in craps than any number, and if you have to pick just one number, it makes sense to choose 7 every time, even though you’re going to lose fairly often.
Getting away from the analogy, there is no certain way to predict how any young player will do. Statistical analysis does not predict a thing (even those who try to predict admit they are fudging the numbers, if you read the fine print). Scouting the player is a little better, but not by much. A combination is useful, but can’t guarantee anything. And it’s even harder to predict personal issues that might derail a player (e.g., Dwight Gooden, who looked like he was on the way to a Hall of Fame career before getting involved in cocaine).
A team tries to take everything into account: statistics, physical ability, personality, etc. They are probably better than average on finding talent, but there is still plenty of room for error. In baseball, for instance, a hitter may not be able to learn to hit a curveball. A pitcher may not be able to learn to throw one. An injury may wreck their career.
At the same time, a borderline player may improve over time. Mike Piazza was drafted in the last round by the Dodgers, and then only because their manager, Tommy Lasorda, was a family friend. Piazza kept working on and improving his game and now he’s finishing up a hall-of-fame career.
It’s especially difficult in baseball because of the long development time. In football and basketball, you have a better idea, since Division I play requires a level that’s close to the pros going in. The colleges are the minor leagues, and you’re just picking the best on what would be the AAA level in baseball. If baseball worked so that the minors were independent and the major leagues drafted from them, the result would be more like those sports.
But it all boils down to judgment, and it’s easy for even the smartest people to misjudge talent. So you keep picking what looks like a 7 to you, since that’s what’s most likely to come up.